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Bridges

 

Overview

"London Bridge is falling down, falling down, falling down." We all know the first verse to that nursery rhyme. But have you ever heard this one: "Set a man to watch all night, watch all night, watch all night"? People once felt that a bridge required a human spirit. They sometimes buried a human sacrifice in the bridge's foundation, so that the spirit could "watch all night." Fortu-nately, we no longer sacrifice a night watchman when we build these structures. But there is still more to bridges than meets the eye. Bridge builders choose from one of several types of bridges or combine two or more. Three basic types are the arch, span, and suspension. The structural elements in a bridge are subjected to various compression and tension forces. Something bearing weight or being pushed together is under compression while something being pulled apart is under tension. In an arch bridge, compression pushes the weight away from the arch and against the side walls and the stones of the arch itself. The Romans were first to build arch bridges, and some of their bridges and aqueducts still stand today. Span bridges consist of beams and/or trusses resting on supports or piers. When the span length would require a very large or heavy beam to support the loads, a truss system is often used. Since a truss is composed of triangles (the strongest polygons because their shapes cannot be distorted), a truss bridge can support heavy loads with its relatively small weight. A suspension bridge hangs from cables firmly anchored at each of its ends. Towers positioned at regular intervals along the span also support it. The main elements of suspension bridges, the cables, are in tension. The trusses hang from the cables and the trusses, in turn, support the deck. Trusses in these bridges provide stiffness to their decks. Suspension bridges are generally used for long spans. Engineers need to accommodatetorsion in their bridge designs. What causes a bridge to twist? Wind. An example of torsion was the Tacoma Narrows Bridge in the state of Washington. A suspension bridge without open trusses, this bridge twisted in the breeze. One day, the bridge twisted so violently in the wind that its center span collapsed. Since then, engineers test bridge models in wind tunnels prior to construction and build their decks more stiffly. When determining what type of bridge to build, engineers evaluate terrain and length of span. In addition, bridges change as our needs and resources change. Engineers constantly look for ways to improve bridge designs and materials. During the recent earthquakes in California, for example, engineers analyzed highway overpasses to find ways to make them stronger.
  • Why is it important to know whether parts of a bridge will be subjected to tension or compression?

Activity

Engineers first create a blueprint and model of a bridge before they begin construction. Models enable them to test the design of their bridges. Often, engineering companies must compete to win a contract. For their presentations, they explain features of their designs with blueprints and models.

Materials

  • graph paper
  • pencils
  • poster board (2' x 3')--one sheet per group
  • scissors
  • glue
  • tape
  • string
  1. Each group will design and build a freestanding bridge for a transportation system of the future. First decide what type of transportation will cross the bridge and what type of bridge you will build. Create a blueprint of the bridge on graph paper.
  2. Using your blueprint, create a model of your bridge from the poster board. Each group is only allowed one sheet of poster board, so measure carefully before you cut. The only other materials you can use in the construction of your bridge are tape, glue, and string.
  3. Present your bridge to the large group. Explain the rationale behind your design.
  4. With all the groups together, test the bridges for length, height, and strength.

    Questions

    For the individual groups:
    1. How did you come up with the initial design for your bridge?
    2. Did your design change as you built your bridge?
    3. Which geometric shapes did you use in your bridge? Why?
    4. How does the strength of the bridge compare to the weight of the bridge?
    5. Would you make any changes in the design of your bridge? For the large group:

Resources

  • Stix, G. (1993, Apr) Concrete solutions. Scientific American, pp. 102-112.
  • TV Ontario videotape: Trussworthy. Landscape of Geometry series. TV Ontario: (800)
    331-9566.
  • Whitney, C. (1983) Bridges. New York: Greenwich House.
  • Wollomir, R. (1994, Jan) Inside the lab and out, concrete is more than it's cracked up
    to be. Smithsonian, pp. 22-31.

Additional sources of information


Design kits such as Structures or The Structures Kit (found in teacher resource
catalogs)
Roebling Chapter of the Society for Industrial Archaeology
c/o Bierce Riley
19 Budd Street
Morristown, NJ 07961

Community resources


Architect
Civil engineer